Exemplary embodiments relate to electronic devices, and more particularly, to memory systems and to methods of operating memory systems.
Semiconductor memory devices constitute a vital microelectronic component commonly found in digital logic systems, such as computers, and in microprocessor-based appliances ranging from satellites to consumer electronics. Therefore, advances in the fabrication of semiconductor memory devices, including process enhancements and circuit-design-related developments that allow scaling to higher memory densities and faster operating speeds, help establish performance standards for other digital logic families.
Semiconductor memory devices are generally classified as volatile memory devices, such as random access memory (RAM) devices, or nonvolatile memory devices. In RAM devices, data is stored by either establishing the logic state of a bistable flip-flop such as in a static random access memory (SRAM), or by charging a capacitor in a dynamic random access memory (DRAM). In volatile memory devices, such as SRAM and DRAM devices, data remains stored and may be read as long as power is applied, but data is lost when the applied power is turned off or otherwise interrupted.
Mask read-only memory (MROM), programmable read-only memory (PROM), erasable programmable read-only memory (EPROM) nonvolatile memory, and electrically erasable programmable read-only memory (EEPROM) devices are examples of non-volatile memory devices which retain stored data even in the absence of applied power. Further, the non-volatile memory data storage state may be permanent or reprogrammable, depending upon the fabrication technology used. Non-volatile semiconductor memories are used as program and microcode storage in a wide variety of applications in the computer, avionics, telecommunications, and consumer electronics industries. A single-chip combination of volatile as well as non-volatile memory storage modes is also available in devices, such as non-volatile SRAM (nvRAM), for use in systems that require fast, reprogrammable non-volatile memory. In addition, dozens of special memory architectures have evolved which contain some additional logic circuitry to optimize their performance for application-specific tasks.
Mask read-only memory (MROM), programmable read-only memory (PROM) and erasable programmable read-only memory (EPROM) nonvolatile memory devices are not freely capable of self-erase and write functionality, and it is thus not easy to update the contents of these memories. On the other hand, electrically erasable programmable read-only memory (EEPROM) nonvolatile memory devices are electrically erasable and writable, and may thus be readily applied to auxiliary memories or system programming memories that require continuous updating.